LED Diagnosis/Troubleshooting
Board 1

LED Diagnosis || Board 2 || Reverse Polarity Damage

First Mixer || VFO || VFO Relays || Bandpass Filters || Crystal Filters

First Single Balanced Mixer

The brightness of the LEDs is a direct indication of oscillator drive. When oscillator drive is introduced, the Gate 2 bias is raised by the incoming oscillator power, and the LEDs light up.

The normal bias of Gate 2 is .05 Volt, due to the 470 resistor going from Gate 2 to Ground. With only .05 Volt bias and no oscillator drive, the LEDs do not light.

Both Gate 1 and Gate 2 are labeled on the PCB for easy identification.

> If the LEDs do not light up, you may have forgotten to place the jumper between VFO OUT (on the VFO section), to the VFO IN (at the first mixer).

>If the LEDs still do not light, use a Frequency Counter and check for a reading at the BOLD square marked "VFO OUT" on the output of the 2nd VFO amplifier.

>If you get a frequency readout, then the VFO is OK but the output may be low. (1) Jumper between the "Stabilizer" box at the output of the first VFO amplifier and the "VFO IN" at the mixer and see if the mixer LEDs turn on. If the LED is on at the 2nd VFO amplifier, check the solder joints at the output of the T4-6T transformer and the connections to the miniature coax cable at both the VFO OUT and VFO IN.
(2) If the LEDs are still dark at the mixer, and the LED for the 1st amp is on, check for a missed solder joint at the transformer or the output cap.

(3) An output transformer (T1-1T or T4-6T) could be bad - a rare occurance. Check VFO troubleshooting below if the mixer seems OK (no solder bridges, no missing solder joints, replacing the 470 ohm resistor with a 100K at Gate 2 lights up the LEDs).

>Lift one end of the 470 ohm resistor connected to Gate 2 (MOSFET on the left hand side) right above the "INPUT IN" box, the LEDs should light up.

If the LEDs do not light, replace the 470 ohm resistor with a 100K resistor, then do the procedure above under the section "MOSFET Amplifiers".

Put the 470 ohm resistor back in the board after running the MOSFET Amplifier diagnosis.

Installation of Proper LEDs

One unusual case of a dead mixer was brought to me at a hamfest. I use tri-color blinking LEDs in one of the display boards. One of these blinking LEDs had been installed in the mixer. Blinking LEDs will not work in the MOSFET circuits because they do not function as a proper ground for the circuit.

>Make sure that the LEDs installed in the mixer are either the smoked red LEDs provided in the kit or the special ordered Bright Reds. Do not use the Super Brights (will not be red colored when powered up) or the mixer gain will be lowered considerably.


>Make sure that you have connected the jumper between the VFO OUT and VFO IN boxes. The LEDs at the first mixer provide diagnosis information on the VFO.

LED Diagnosis

If the LEDs at the First Mixer are off and the LED at the First VFO Amplifier is slightly dim, then the VFO has no output. If you have a sensitive Frequency Counter, check for output at the 3.3pf capacitor between the VFO and the First VFO Amplifier.

>If all the soldering looks good, there are no solder bridges, and no frequency reading at the 3.3pf cap, then the 2N5486 is blown. Care has been taken in designing the board to prevent the 2N5486 from being blown, but touching the VFO toroid coil with a metal screwdriver when the VFO is powered up will sometimes blow the 2N5486.

>If the LEDs at the First Mixer are dim in comparison to the Post Mixer Amplifier, the LED at the First VFO Amplifier is bright, and the Second VFO Amplifier LED is slightly less bright, then check the output transformer of the First VFO Amplifier. Check for a bad solder joint on the secondary side of the transformer or a bad transformer.

One builder had the output jump to normal and then back to half power when the board was flexed. The T1-1T transformer was cracked and had an intermittent open on the secondary winding. The problem was found when a pair of pliers was put across the transformer and squeezed slightly to see if a pin was loose, the transformer instantly broke apart. The T1-1T was replaced and the problem was solved.

>In normal operation, the Second VFO Amplifier LED will be slightly brighter than the First VFO Amplifier.

>If the Second VFO Amplifier LED is slightly brighter than the First VFO Amplifier, and the LEDs at the First Mixer are off check:
(1) for a bad/missing solder connection at the secondary of the T4-6T
(2) a bad transformer (note paragraph above)
(3) a shorted jumper cable at the VFO OUT box (2nd VFO amplifier)
(4)a shorted jumper cable at the VFO IN box at the First Mixer

Improper Frequency and Changing Frequencies

>If the VFO frequency is extremely high, check for a short to ground at the stators of the main tuning capacitor.

>If the frequency is slightly above 15 MHz, take one winding off the toroid coil. If the frequency will not reach 13.5 MHz (SWL version) or 14 MHz (Ham Band version), add one winding. Errors outside this range may indicate a capacitor in the wrong location.

>Check the capacitors in the feedback circuit and make sure the poly capacitors are paired 100pf and 120pf (on each side of the 2N5486). The feedback capacitors are marked 82 NPO (orange color) and 39 NPO (Blue color) between the 2/8 Ceramic trimmer and the 2N5486.

Note: Do not change the feedback and poly capacitors when making changes inside of the normal range of the VFO (9 MHz to 15 MHz). When going outside the normal range, change the feedback capacitors first, then the polys if needed.

Slight flex of board, tuning the main tuning cap, or touching the VFO circuitry - Improper Frequency, loss of VFO signal (1st mixer LEDs go out), and Changing Frequencies - most likely on 40 and 20 meters.

>Check the relay pin to ground on the 10.545 VFO relay. The coil pins are the two center pins and the one to look for is the pin soldered to the ground plane. If this pin has been soldered incorrectly the VFO will lose signal and show erratic frequencies when tuning the VFO or touching the PCB.

>Sometimes the problem will go away after ten minutes of warm-up if the pin has broken from the solder. With a magnifying glass you will see a very small gap between the pin and solder.

>Touch up the solder joints going to the center two pins of both VFO relays, especially if it occurs on 20 meters.

>The pins of the relays do not accept solder very well. Sometimes the solder at the joint needs to be removed, the pin cleaned with a razor blade or some sharp instrument, tinned, then resoldered if the problem persists.

VFO Relays

Voltage or current problems

Do the following diagnosis with the left hand switch (Bandpass Filters) on the 20/17 side and the right hand switch (Crystal Filters) on the 40/20 side. With these switch settings, the receiver will be on 20 meters.

If the 3.547 crystal filter LED is on, and the 10.455 relay does not turn on when the bandpass filter is on 20 meters, short the "C" and "E" pins of the phototransistor at the VFO amplifiers and see if the relay turns on.

If it does, then check two things:

>First, there may not be enough IR energy hitting the BPX38-4 to turn on the VN0106N3 enough to supply the correct current. When first testing a new board, most of time I have forgotten to aim the IREDs! Find the IRED at the 3.547 crystal filter and check for proper alignment. Also, check to make sure the black tube is straight.

>Second, the receiver voltage may not be high enough. The power supply for the receiver must run 12 to 13 volts. If it runs near 11 volts, the switching on the board may not work.

If the 10.455 relay/LED does not turn on check the following:

>Check the voltage at the "A" terminal of the photodiode near the 10.455 relay. The voltage should read above 9 volts.

>If the voltage is around 4-5 volts, remove the tubing from the phototransistor between the VFO amplifiers and shine a light directly on the phototransistor. The voltage should be around 10 volts.

>If you do get 10 volts, then not enough IR energy is getting to the phototransistor to supply enough current for the VFO relays. Check to make sure the black tubing is straight. Any bends will block some of the IR from the IRED at the Crystal Filter and not fully turn on the phototransistor.

Also check the soldering for the leads of the RF Choke and resistor feeding voltage to the IRED at the Crystal Filter. Also check the leads of the IRED.

>Second, if you do not get 10 volts, check the VN0106N3 as per the instructions below.

A large RF choke with very low resistance should be placed at the 12 Volt connection to the board going to the supply trace for the BPX38 and relays. Three large low resistance RF chokes are supplied in the kit; one is recommended for this location.

Note: The RF Choke in the Rev 10/1/2005 boards at the 12 Volt connection box was removed and a large one was placed halfway down the supply trace next to the poly caps at the VFO.

The same type RF choke can be placed at the RF choke position next to the poly caps at the VFO. This will lower the resistances in the 12 Volt supply line and raise the voltage level. This is usually not necessary.

The value of the RF choke is not critical. Any value from 10uH to 1mH can be used. A choke can be wound on an FT50-43 or FT37-43 ferrite core, with 10 to 20 turns, whatever will fit with the wire you have available.

If you have nothing available, short the chokes with wires until RF chokes can be ordered.


When neither of the LEDs at the VFOs will light, the most likely suspect is the VN0106N3.

The VN0106N3 is blown by reverse polarity and shorts on the Source pin. If any shorts have occurred on the 12 Volt line from the Source pin to any of the relays or around the 10.455 MHz relay or the IRFU220, the VN0106N3 could be blown.

To test for a blown VN0106N3, measure the voltage at the source pin when the IR LED at the 3.547 MHz crystal filter is on. The voltage will be zero with a blown VN0106N3.

Broken Pins

The problems encountered will be the relay is not switching (cannot here it click when it is turned on), or the VFO will stay at 14 MHz when the receiver is switched to 40 or 20 meters. Or when switched to 20 meters will stay at 10.545 MHz.

Another problem will be an unstable VFO frequency. Sudden shifts in frequency either small or large. Or it may be misdiagnosed as light effecting the VFO through the photo sensitive devices.

When the VFO frequency stays at 14 MHz, the problem is at the 10.545 relay. The 10.545 LED is not in the circuit of the relay so it will not indicate a broken power pin. (Also check the VN0106N3 and proper IR energy.)

If the VFO frequency stays at 10.545 MHz the problem will be the 10.455 relay. The 10.455 LED is in the circuit of the relay coil and will not lite (or flicker) if a power pin is broken.

Make sure the relays are mounted flush with the PCB. If a relay can be pushed back and forth easily it is highly likely that one or two pins may be broken underneath the relay.

>One quick way to determine a broken pin is to resolder the pins and try to pull the pin from the board. The broken pin will be loose and stop the frequency changes or stop the relay from functioning.

>Check for proper voltage going to the ungrounded coil pin (two pins in the center of the relay).

>If approximately 9 volts is at the coil pin and the relay is not switching or the VFO does not change frequency, remove the relay to check for broken pins.

Email me if you need another relay.

Bandpass Filters

Bandpass Filter Pot

Incorrect voltages at the center solder lug

Check for a short to ground on the outside soldering lugs of the pot underneath the board. The lowest reading that should be seen is 1K.

All the resistors around the "BFP" circle in the middle of the bandpass filters are 100K. The 100K resistors provide RF choke action to keep RF from getting into the 12 Volts. Check to make sure there are no 1K or 100 ohm resistors soldered in that location.

Bandpass pot does not work and dead receiver

If the bandpass filter pot does not tune a peak on any band, and the receiver sounds absolutely dead, check for proper orientation of the 2N3019 in the RF amplifier. Make sure the tab is properly placed according to the PCB footprint of the transistor.

Incorrectly inserting the 2N3019 does not always destroy the transistor. If you have a DVM with a transistor checker, the beta reading should be 120 to 130, NPN position.

With the diode checker on a DVM (if a transistor checker is not on the DVM), put the positive lead on the base, and touching the negative lead first on the collector and then the emitter, both readings should be close to .780 volt. No reading or a short indicates a blown transistor.


Most errors in the filter sections are improperly soldered coils, or a bad solder joint at a pin of the relays.


If the wire of a coil is pushed too far down into its soldering hole, going past the cleaned part of the lead, a bad solder joint is made.

>Use a VOM and measure across the coils at a point away from the soldering holes. The VOM should read zero resistance.

>If an LED does not light, check the inside pins of the relay.


>If you are not getting signals when the LED is on, check the outside pins of the relay.

The pins of the relays should always be tinned before soldering to the board. Make sure solder completely surrounds and flows onto the pin.

>If you have a stubborn pin, use a solder sucker, remove the solder from the pin, scrape the pin clean, carefully, with the sharp tip of a razor blade (or utility knife), and resolder.

>Place a VOM in resistance mode, and measure between the output cap of the 50 ohm pad and the .01 cap beneath the 1 Meg resistor, to make sure the relay is being activated when the LED is being turned on.

>If the relay is not turning on, make sure the voltage feeding the receiver is at least 12 volts.

If you suspect a broken pin, especially if a relay can be easily pushed back and forth, check VFO Diagnosis, Relays, Broken Pins, above.

IREDs and LEDs

>If both LEDs are on, shield any light that may be hitting the Photodiodes. One of the LEDs should turn off.

Shining a bright shop light on the Photodiodes is a test for correct function of the Photodiodes. If an LED doesn't turn on when light is on its Photodiode, short across the terminals of the Photodiode on the top side of the PCB. If the LED lights, then the Photodiode is bad (a rare occurance). If the LED doesn't light, check the following:
(1) a bad solder joint on the pins of the Photodiode
(2) LED soldered in backwards
(3) IRFU220 in backwards or a bad solder joint

>If the Bandpass Switch is on the 20/17 position and the 40/30 LED is turned on, check the IR Path of the 20/17 IR LED and see if any Infra-red light is leaking onto the 40/30 Photodiode. Use a small piece of paper as a barrier to block IR to the other side.

>If the 20/17 side of the Bandpass filter stays on regardless of the position of the Bandpass Switch, look for a short to ground at the 20/17 side of the Switch. The Switch pad is very close to the ground plane on the front side.

>The coils of the 40/30 Bandpass Filter should be mounted perpendicular to the footprint to block any IR coming from the 20/17 IR LED to the 40/30 Photodiode.

>If an LED does not come on when it is selected by the Bandpass Switch, realign the IR LED and Photodiode to make sure they are aimed directly at each other. Bending the wires of the IR LED close to the PCB, so it can be bent closer to the Photodiode, will help.

The 20/17 side of the filter should have the IRED as close to the photodiode as possible. Also, the 20/17 input relay will not always turn on when the receiver voltage is near 11 Volts. The reason is two IR LEDs are activated through the output relay of the 20/17 Bandpass Filter, and a full 12 Volts is needed to insure enough output of the IREDs.

>Check the 1 Meg resistors to the left hand side of the Photodiodes (immediately below the LEDs) to make sure they are the correct value. If a 100K or lower value resistor was accidentally placed here, the LEDs will not turn on.

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Crystal Filters

Switching Circuit

>The LEDs at the crystal filter are controlled by the pair of VN0106N3's to the left of the crystal filters. The most common problem is a missed solder joint at one of the resistors that are a part of the circuit.

>If both LEDs are on at the same time, one of the VN0106N3's could be bad. Also, a missed/bad solder joint at the 100K resistor could be causing the problem (located between the VN0106N3's).

IREDs and LEDs

>If you are not getting enough IR energy to activate the Phototransistor at the VFO Amplifiers, lower the 470 ohm resistor (to the left of the IR LED on the 40/20 side) to 220 ohm. This will only add 1dB of loss to the filter, which will hardly be noticeable.

The maximum current that the IR LED can handle is about 50 MA. The 220 ohm resistor should have a wattage rating of 1/2 watt or 1 watt.

>The same thing can be done to the IR LED at the 30/18 crystal filter, if more IR energy is needed.

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MOSFET Amplifier/MOSFET Device Diagnosis

Board 2 Diagnosis/Troubleshooting

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Last Update: 7/31/08
Web Author: David White